To unlock the Legacy Restored trophy in Assassin's Creed Odyssey, you will need to upgrade the Spear of Leonidas to level 6.
Relatively early on in the game, you will learn how and where to upgrade the Spear of Leonidas using Artifact Fragments taken from Cultist members once you have killed them.
Each level will require an increasing number of Artifact Fragments to upgrade. The numbers required are below:
Level 2: 1 Artifact Fragment
Level 3: 3 Artifact Fragments
Level 4: 7 Artifact Fragments
Level 5: 9 Artifact Fragments
Level 6: 22 Artifact Fragments
Once you have acquired the number of Artifact Fragments needed for the next upgrade, you can fast travel back to Andros Island, and enter the Ancient Forge and upgrade at the large stone in the back of the cave.
Once you have reached level 6, the trophy will unlock.